Our dr. Richard besser. And a guy named dr. Derek shepherd, also known as patrick DEMPSEY, ALSO KNOWN as McDreamy of abc's hit show "grey's anatomy." The two men hit the road. And dr. B. Got quite a workout. Reporter: For ten seasons, dr. Derek shepherd has made ladies swoon. I dissed derek. What do you do on your bad days? Reporter: With that one line, actor patrick dempsey became McDREAMY. When I was invited to ride side-by-side with another, well, doctor, in his home of lewiston, maine, I jumped on it. And I found the charity that MAKES dempsey McDreamy offscreen is the dempsey center. What is the dempsey center. If you're diagnosed with cancer, you yourself or someone in the family, we are there to help you go through that journey. And all of the services are provided for free. That's why we're doing the challenge. That's our way to raise money. Reporter: That challenge is the dempsey challenge. An annual run, walk and cycle fund-raiser. This year, I was honored to be asked to pedal along. But this ride -- I'm coming. Was just preparation for the big day. Cycling 50, yes, 50 miles. First time I've done that. Feel the burn there? Reporter: Yep. The center, founded in 2008, came about for a very personal reason. Dempsey's mom, for over ten years, has battled through 11 bouts of ovarian cancer. When your mom was first sick, there wasn't thinking like this. There was nothing like this. And I thought I need to give back to the community. Reporter: Dempsey lives in los angeles. But chose his home turf as a way to pay it forward. I had some advice when i left, never forget where you come from. And what does that mean, really? And you start to explore it. That's when you find yourself again. We're going to use a lateral -- Reporter: And something i just had to ask. Doctor-to-doctor? Okay. Reporter: Do people ask you medical questions? Sometimes they do. People are like, you know what to do here. I have no idea what to do. I just play one on television. Reporter: I got to tire my legs out is inaccurate. Using his fame to make cancer a little easier to bare. Everywhere you go, you're McDREAMY. Is it too much? I know I'm going out. And therefore, it's my responsibility to say hi and take the time. I've sometimes met someone who has been famous and they weren't the nicest. And I think that's crushing. Reporter: And of course, we crushed the bike-a-thon. Another reason he has it in maine, has to be the lobster. Can I try your butter? Sure. He's such a nice guy. Absolutely. And he gave dr. Besser some lobster. So, it's all good.

This transcript has been automatically generated and may not be 100% accurate.

{"id":21048567,"title":"McDreamy's Passion Project: Patrick Dempsey Bikes for a Cause","duration":"3:00","description":"\"Grey's Anatomy\" actor rides in support of Dempsey's Center for Cancer Hope and Healing.","section":"GMA","mediaType":"Default"}